Method for laser-cutting structural components to be joined

ABSTRACT

In a method for cutting structural components to be joined by laser radiation that is guided by a computer-controlled manipulation system provided with a nominal path for each structural component corresponding to a joining line that is curved as a result of at least one of the structural components being three-dimensionally shaped, the structural component surfaces of the structural components including the joining line are determined measuring technologically. Based on measuring results, the nominal path corresponding to a penetration line of the structural components to be joined is calculated. A marking is formed on a first one of the structural component surfaces. When performing the cut on the structural component surface provided with the marking, the resulting cutting gap and the marking are determining measuring technologically. When a deviation of the cutting gap from the nominal path is detected, the manipulation system is controlled to correct the deviation.

The invention relates to a method comprising the features of thepreamble of claim 1.

DE-Z TechPress 1/97, pp. 21 through 26, discloses a method for cuttingstructural components to be joined by means of laser radiation, whereina cutting head of a laser cutting device emitting laser radiation isguided by means of an automated manipulation system. Guiding is carriedout along a predetermined nominal path.

When cutting is carried out in order to produce structural components tobe joined, the nominal path must correspond as precisely as possible tothe joining line. When the structural components are to be joined atcurved joining lines, for example, by means of welding with high-energyradiation, such as laser radiation, the manipulation system duringcutting of each one of the structural components to be joined musteffect a path of the laser radiation which corresponds as precisely aspossible to the curved joining line. A plurality of error sources canimpair the precision. The shaping of the structural components to bejoined and the path precision of the manipulation system should bementioned as examples. Moreover, the method should provide a productionoutput as high as possible so that high cutting and welding speeds aredesired. The quality of the welding connection, however, should notsuffer so that the cutting gap width and the welding seam width must bemaintained within predetermined limits. For the aforementioned reasons,it is required to cut the structural components to be joined asprecisely as possible.

DE 38 18 402 C1 discloses a method for torch cutting where thepenetration line of two bodies is measured based on their actualdimensions. For this purpose, a predetermined area is assigned to bothbodies, respectively, which area comprises the penetration line.Measured values of both areas are saved and, based on these values, thepenetration line is calculated according to which the welding seam isthen produced. The use of a computer-controlled manipulation system ininteraction with a laser beam is not disclosed. During cutting nocorrection whatsoever of the torch cutting machine is carried.

U.S. Pat. No. 5,463,202 describes a method for processing workpieces bylaser radiation wherein secondary light originating at the work locationis determined and supplied to a control device that loads an NC deviceprovided for controlling a worktable supporting the workpiece. Guidingof the laser radiation by means of a computer-controlled manipulationsystem is not carried out.

The invention has therefore the object to improve a method of theaforementioned kind in the sense of a precise cutting to size.

The object is solved by the features of the characterizing portion ofclaim 1.

It is important in connection with the invention that at least thesurface sections of the structural components are measured which are tobe provided with the joining line. As a result of this, structuralcomponents which are not precisely known with regard to their geometricshape can be precisely determined. Based on the precision of themeasuring results, a penetration line can then be calculated that iscommon to both structural components to be joined. The nominal pathcorresponds to this penetration line and can thus be preciselydetermined by computation. Imprecisions as a result of apremanufacturing step of the structural components have no effect on theprecision of the cut because of their measuring-technologicaldetermination. Moreover, for the invention it is important that amarking of the structural component surface that has beenmeasuring-technologically determined is carried out. Such a markingserves for referencing that must be performed so that a sensor duringcutting of a structural component can detect the resulting cutting gapwith regard to its size and position in an error-free way, i.e., free ofcorrelation errors between the detected cutting geometry and the nominalpath. The measuring-technological detection of the resulting cutting gapcan thus be precise enough in order to control the manipulation systemin the case of a deviation of the cutting gap from the nominal path suchthat a correction of the deviation is achieved during cutting.

The method is carried out such that the marking of themeasuring-technologically determined structural component surface iscarried out simultaneously with its measuring-technologicaldetermination. Since marking is carried out during the measuringprocess, marking errors are avoided which could occur for a time-delayedmarking, for example, because of the time delay of different measuringand marking parameters.

It is preferred that marking is carried out by the laser carrying outthe cutting of the structural component. It is therefore not to beexpected that errors will occur during marking which could result fromspatially different positioning of the laser and a special markinginstrument. Also, errors are avoided which could result from differentdynamic behavior of the laser and a marking element.

Moreover, it is preferred that the marking is carried out on the nominalpath or in its vicinity. This primarily achieves that the area to beobserved can be kept small. This is advantageous in order to be able toemploy sensors of a less complex configuration and in order to keep thecomputation expenditure minimal. Moreover, this provides the possibilityof operating at high cycle times for a predetermined computationexpenditure; this is advantageous for a precise correction of deviationsduring cutting or enables a higher cutting speed.

In the above described sense, it is moreover advantageous when themarkings are formed only at some locations.

The aforementioned fast correction or a time-optimized performance ofcutting can be improved in that the marking is in the form of thecutting gap that is produced during cutting. However, this requires asufficiently reliable path behavior of the manipulation system, forexample, with regard to kinetics, dynamics, and thermal effects so thata tolerable cutting gap width will not be surpassed. The same holds trueto a lesser degree also for the aforementioned method variant whereinthe markings are carried out only at some locations. The reliability ofthe path behavior entails that referencing of the nominal path isrequired only in intervals.

The method can be improved in that the same sensor is employed for themeasuring technological detection of the marking and of the cutting gap.This simplifies not only the configuration of a device that is requiredfor performing the method but also avoids errors which are to beexpected as a result of a complicated configuration of the device withdifferent sensors, for example, because of a different arrangement and,accordingly, different measuring positions of several sensors.

When performing the method, it can also be advantageous when a sensoremployed for a measuring technological determination is an imagingsensor. An imaging sensor facilitates particularly, on the one hand, toenable with minimal constructive expenditure simultaneously themeasuring technological determination of the structural componentsurface and its marking as well as the detection of this marking and ofthe effective course or the resulting cutting gap.

In order to employ a proven technology in the method, it is proposedthat the sensor for the measuring-technological determination is a CCDcamera. The data recorded with the CCD camera can be processed withproven image evaluation methods and can be integrated into alreadyexisting computers and control devices. Instead of a CCD camera, otherimaging sensors can be used which are, for example, a CMOS camera or aPSD sensor.

In order to obtain a reliable measuring result of two or severalstructural components with regard to their relative position in thejoining situation, the method can be performed such that the structuralcomponents for a measuring-technological determination of theirstructural component surfaces are positioned in a predetermined relativearrangement. This predetermined relative arrangement can be adjusted, inparticular, relative to their future joining position, wherein onestructural component can have a position which it later would assume forjoining while another structural component or several other structuralcomponents must be removed from their joining position only so far asrequired by the structural component rests to be cut off, as long as themanipulation device enables such an arrangement without undesirableinterference of the remaining method sequence.

For performing a measuring technological determination of two structuralcomponent surfaces, it can be advantageous to perform the method suchthat for a measuring technological determination of two structuralcomponent surfaces two distance measuring devices are used that arepositioned at a predetermined relative arrangement to one another. Therelative arrangement of the distance measuring devices to one anothercan be predetermined such that it is adjusted to the relativearrangement of positioning of the structural components for themeasuring-technological determination of their structural componentsurfaces according to the afore described method. The predeterminedrelative arrangement of two distance measuring devices enables moreoverthe simultaneous detection of two structural component surfacesincluding optionally also the formed markings provided thereon.Therefore, errors cannot occur which possibly are to be expected for anarrangement of a single distance measuring device when it issequentially aligned relative to both structural component surfaces andemployed for a measuring technological determination.

The method according to the invention will be explained in more detailwith the aid of the illustration in the Figures. It is shown in:

FIG. 1 a schematic arrangement of all components required for performingthe method;

FIG. 2 an illustration for determining a nominal path;

FIG. 3 an illustration for explaining the transfer of data of ameasuring technologically determined structural component surface and ameasuring-technologically determined marking on an imaging sensor;

FIG. 4 a schematic illustration of a cutting gap produced duringcutting; and

FIG. 5 a calculated structural component surface that is used forforming the cutting gap.

In FIG. 1, two structural components 1, 2 are schematically illustrated.Both are illustrated as being tubular; however, the tubularconfiguration is not important—in place of it, other three-dimensionalstructures are also suitable. In particular, one of the structuralcomponents 1, 2 is three-dimensional in such a way that athree-dimensional curved joining line results when the two structuralcomponents 1, 2 are to be connected to one another by welding.

The embodiment of the structural components 1, 2 as regards their shapeis as needed, for example, structural components can be employed whichare used for filling truss structures wherein butt seams, for example,between pipes or profiled sections used as frames and, for example, flatelements as filling structures are welded together. Also, IHT structuralcomponents can be used which, are produced by internal high-pressuretransforming and, in connection with space-frame concepts, are welded bymeans of buckling arm robots. In the context of space-frame concepts,structural components made of steel can be produced also. Also,structural components of easily deformable materials can be employed,for example, aluminum. The connection between these structuralcomponents is produced by welding and without, for example, preparativework such as edge-forming and without forming during the flying process,for example, by pressing for the purpose of a shape-transformingadaptation. The aforementioned structural components are used, forexample, in the automotive industry, in prototype construction, as wellas custom-made constructions.

The structural components 1, 2 are positioned in FIG. 1 in apredetermined relative arrangement wherein the structural component 1 isin a fixed position that during cutting and during joining remainsunchanged. The cutting step is carried out for the purpose of a spatialadaptation of the structural component 1 to the outer circumference ofthe structural component 2 wherein the latter, according to FIG. 2, canbe moved from a position A by a predetermined displacement V into aposition E at the end face 5 of structural component 1 in order toperform the joining step.

In order to be able to match the end face 5 of the structural component1 to the outer circumference of the structural component 2 as preciselyas possible, the structural components 1, 2 must be precisely known atleast in those structural component areas which are to be joined. Forthe structural component 1, the structural component surface T01 isschematically indicated here, and the structural component surface T02of the structural component 2 is indicated. In the area of thesesurfaces T01, T02 the joining line is to be formed.

Positioning of the structural components 1, 2 can be combined with itsmanufacture. The structural components 1, 2 are manufactured, forexample, by shaping. In this connection, the structural components arefixed in position, for example, by fixation bolts of the shaping toolduring the shaping process. The fixation bolts engage reference bores ofthe structural components 1, 2 in order to secure the position of thestructural component. By means of the reference bores the arrangement ofthe structural component 1, 2 is defined. In the initial positionillustrated in FIG. 1, the tailoring cut is performed on the structuralcomponent 3, and in the end position of the structural component 2,which, according to FIG. 2, can be derived from the indicateddisplacement from A to E, a predetermined gap width between the twostructural components 1, 2 should not be surpassed. The acceptable gapwidth between a cutting edge of the structural component 1 and of theouter circumference of structural component 2 is determined by thecapability of the welding arrangement for bridging such gaps. ModernNd:YAG lasers can bridge gap widths up to 100 μm.

In order to be able to determine the configuration of the structuralcomponents 1, 2, a measuring device M is provided. By means of themeasuring device at least those structural component surfaces T01 of thestructural component 1 and T02 of the structural component 2 must bedetermined measuring technologically in which the joining line is to beformed, i.e., where the structural components 1, 2 are to be welded toone another. It is assumed in this connection that the structuralcomponents 1, 2 are positioned in a predetermined relative arrangementrelative to one another as illustrated schematically in FIG. 1. Underthese conditions, two distance measuring devices A1, A2 can be used thatare also positioned in a predetermined relative arrangement to oneanother determined by the angle size φ in FIG. 1. The distance measuringdevices A1, A2 have measuring lasers which can generate on thestructural component surfaces T01, T02 a linear focus L1, L2,respectively. When carrying out the distance measurement, surface scanscan be performed for generally known measuring methods in differentways. Measurement can be carried out by triangulation, holography, orinterpherometry. Different measuring signals can be used, for example,in accordance with white light or radar interpherometry. Differentdetection devices can be used, for example, CCD cameras, CMOS cameras orPSD sensors. The obtainable precision is estimated, for example, in thecase of the triangulation method, to be one per thousand. Theinterpherometry methods can achieve a precision within the micrometerrange. The actual precision depends, however, strongly on the boundaryconditions, for example, soiling of the structural components to bemeasured.

By means of the measured results for the structural component surfacesT01, T02 comprising the joining line, the penetration line D of thestructural components 1, 2 can be calculated when talking intoconsideration the displacement of the structural component 2 from A to Eso that the structural component surface T02 is in the displacementposition T02V illustrated in FIG. 2. The penetration line D istheoretically that position which the joining line assumes in the idealsituation and corresponds thus to a nominal path SB, as illustratedschematically in FIG. 5. The nominal path serves for controlling thecutting process in the case of the concrete structural components 1, 2and can be different for a different structural component pair made byan identical manufacturing processes.

For performing cutting of the structural components 1, 2, a cuttingsystem S is provided. It is comprised essentially of a manipulationsystem with a laser and can be provided with the measuring device M. Asa manipulation device, all known systems can be used, for example,buckling arm robots. The system comprises a beam deflection system,characterized in FIG. 1 by a rotary mirror DSp, to which rotary mirrorDSp a laser beam emitted by a laser is guided via a deflection mirrorSp. The rotary mirror can be comprised of dichroitic material whichreflects laser radiation coming from a Nd:YAG laser and accordinglydeflects it onto the structural component. In FIG. 1, the double arrow 6indicates that the rotary mirror DSp is able to perform beamdeflections. This movement can be carried out independent of possiblyoccurring movements of the manipulation system, i.e., in additionthereto. In this way, imprecise movements of the robot axes duringcutting can be compensated, for example, during cutting of contours withsmall radii. In this way, the path precision is increased. Within therange of movability of the rotary mirror DSp, movements of robot axesduring cutting can also be prevented entirely.

For performing the afore described cutting process, the cutting system Sis provided with a sensor 4. The sensor 4 is used for themeasuring-technological determination of the surfaces T01, T02. Duringthe measuring technological detection for determination of the surfaceof the structural component 1, the sensor 4 detects the image BI1 of theline focus L1. As a result of this, by employing the measuring resultsof the distance measuring device A2 and by means of the computer (notillustrated) that knows the nominal path SB, the sensor 4 is able todetect and measure the penetration line or the joining line duringwelding. Accordingly, it is possible to monitor the joining line as wellas the cutting gap of the structural component 1 during cutting by meansof the sensor 4 by referencing the nominal path SB.

The sensor 4 is an imaging system, i.e., a CCD camera. The CCD cameraobserves particularly the structural component surface T01 to bedetermined measuring technologically. Required for the measuringtechnological determination is that the signals of the process arereceived and transmitted to the computer (not illustrated) where theyare processed. In the imaging method, the viewing orientation of thecamera can be important. The viewing orientation of the CCD cameraaccording to FIG. 1 is coaxial to the laser beam. With regard to this,the rotary mirror DSp is able to allow penetration of the secondaryradiation reflected on the structural component 1. The sensor 4 can thusdetermine heat radiation emitted by T01 because it is not reflected bythe rotary mirror DSp. As a result of this, it is possible to observe awelding seam during joining or a cutting gap 3 during cutting of thestructural component by means of the CCD camera or the sensor 4.

In order to ensure a precise determination of the structural componentsurface T01, it is necessary to reference the CCD camera. Referencing iscarried out by providing on the structural component 1 a marking Ma onthe structural component surface T01 at a predetermined relativearrangement of the structural component 1 and of the manipulation systemor the cutting system S. The marking Ma is generated by means of theprocessing laser that is also used for cutting the structural component1. The resulting reflected radiation 7 penetrates the rotary mirror DSpand generates on the sensor 4 the image BMa of the marking Ma. Since thespatial arrangement of the measuring system M and of the cutting systemS, on the one hand, and of the structural component 1, on the otherhand, are known, the image BL1 of the line focus L1 therefore has aknown spacing from the image BMa of the marking Ma; this is theprerequisite for the above-mentioned computational precise determinationof the penetration line D or the joining line and also of the nominalpath SB. The computer has available in the predetermined range theprecise position Ma′ of the marking Ma. By means of observing themarking Ma, it is therefore always possible to determine which actualposition the cutting gap 3 or a joining line has on the structuralcomponent 1. By means of the sensor 4, it is thus always possibleaccording to the standard of the reference image 4′ according to FIG. 5to provide a precise monitoring of the actual processing step, forexample, of the cutting step. The nominal path SB is always known withregard to its location P and its directional vector R so that thepresence of these parameters P, R during machining can always bemonitored. In the case of a deviation from the nominal path SB or of theaforementioned parameters P, R during processing, a control of themanipulation system in the sense of correction of this deviation iscarried out.

The afore described method and its variant serve for a precise cuttingof a structural component as a preparation for a precise joining with asecond structural component and as a preparation for the subsequentwelding along a joining line precisely determined by the cutting step.At least one of the structural components has a three-dimensional shapewhich is however not precisely known beforehand. As a result of this,the cutting contour and the slant of the cutting edge are notpredetermined with the required precision. As a result of the unknowngeometric shape of the structural components to be joined and as aresult of the unknown position of the manipulation device for guiding aradiation source or for guiding the laser beam, during machiningcorresponding imprecisions will be caused that must be measured. Eventhough the nominal path for processing is not predetermined and must bemeasured, and even though the required upper limit for the deviationfrom the nominal path of the joining gap or of the cut edge resultingfrom cutting is smaller than the path deviation of the controlledmanipulation system, with which the cutting tool and the measuringsystem are guided, predetermined precisions of, for example, less than100 μm joining gap width can be observed. Path deviations between thecutting gap and the nominal path are determined at a high repetitionrate during the cutting process and determined within predeterminedlimits independent of the position and of the movement of themanipulation system.

Quality and output of the imaging system are determined by the level ofthe spatial and temporal resolution. A high spatial resolution enablesan enlargement of the image section and thus an even more precisecontrol. A high temporal resolution enables higher cutting speeds andthus a higher production, even for complex structural componentcontours. The precision can be increased such that, for example, overlapseams or edge-formed seams requiring an increased expenditure for seampreparation and having a reduced strength in comparison to butt seamsare no longer needed.

It is important that during the measuring process, i.e., during themeasuring technological determination of the structural componentsurface T01, a marking is generated. Above, the formation of a singlemarking Ma has been described. Markings, however, can also be repeatedwhen a complex geometry of the structural component 1 requires this.Such markings are generated along the nominal path or in its vicinity sothat it can be easily observed with the imaging system. Such markingscan be referred to as a measuring technologically detected link by whicha repeated or continuous referencing of the sensor 4 is possible.

1-11. (canceled)
 12. A method for cutting structural components to bejoined by laser radiation that is guided by means of acomputer-controlled manipulation system, wherein the manipulation systemhas preset a nominal path for each structural component, wherein thenominal path corresponds to a joining line that is curved as a result ofat least one of the structural components being three-dimensionallyshaped, the method comprising the steps of: a) determining measuringtechnologically structural component surfaces of the structuralcomponents to be joined, which structural component surfaces comprisethe joining line; b) calculating, based on measuring results obtained inthe step a), the nominal path corresponding to a penetration line of thestructural components to be joined; c) providing a marking on one of thestructural component surfaces; d) performing a cut on the structuralcomponent surface provided with the marking; e) when performing the cut,determining measuring technologically a resulting cutting gap and themarking and comparing the cutting gap and the nominal path, using themarking as a reference; f) when a deviation of the cutting gap from thenominal path is detected in step e), controlling the manipulation systemto correct the deviation.
 13. The method according to claim 12, whereinthe step c) and the step e) are carried out simultaneously.
 14. Themethod according to claim 12, wherein the marking is formed by a laserperforming the cut in step d).
 15. The method according to claim 12,wherein the marking is formed on the nominal path or in the vicinity ofthe nominal path.
 16. The method according to claim 12, wherein themarking is formed only at some locations.
 17. The method according toclaim 12, wherein the marking is the cutting gap.
 18. The methodaccording to claim 12, wherein in the step e) one and the same sensor isused for determining measuring technologically the marking and thecutting gap.
 19. The method according to claim 12, wherein in the stepa) and the step e) an imaging sensor is used for measuringtechnologically determination.
 20. The method according to claim 19,wherein the imaging sensor is a CCD camera.
 21. The method according toclaim 12, wherein in the step a) the structural components arepositioned in a predetermined relative arrangement to one another. 22.The method according to claim 12, wherein in the step a) two distancemeasuring devices are employed and are positioned in a predeterminedrelative arrangement to one another.